Biosensing instruments that employ disposable sample strips enjoy wide consumer acceptance. Such instruments are employed for the detection of analytes such as glucose and cholesterol levels in blood samples and, in general, provide accurate readings if the user is careful to follow the instrument's directions. More often than not, however, the user is careless in the use of either the sample strip or the instrument and erroneous readings result. Accordingly, significant efforts have been taken by instrument manufacturers to reduce the potential for errors during the use of sample strips and instruments.
Even if a biosensing instrument and sample strips are employed properly, the presence of a manufacturing defect in either will cause erroneous readings. Thus, while great care is taken in the production of such instruments and sample strips, there is a need to incorporate analytical procedures in the instrument that enable instrument malfunctions, sample strip irregularities, and user errors to be detected so as to prevent erroneous analyte readings.
The prior art includes a number of disclosures of biosensing instruments that employ disposable sample strips. In U.S. Pat. No. 5,108,564 to Szuminsky et al., a biosensing instrument is disclosed that measures glucose concentrations in blood. The instrument depends upon a reaction wherein glucose, in the presence of an enzyme, catalyzes a reaction of potassium ferricyanide to potassium ferrocyanide. After the reaction has completed, a voltage is applied across a reaction zone and causes a reversal of the reaction with an accompanying generation of a small, but measurable current. That current is termed the Cottrell current and, in dependence upon the concentration of glucose in the reaction zone, follows a predetermined curve during the reverse reaction. A reading of the Cottrell current is converted into an indication of glucose concentration. The instrument senses an impedance across the reaction zone and determines when a blood sample has been emplaced therein by detecting a sudden change in current flow. At such time, an incubation period is commenced, followed by application of a potential across the reaction zone and measurement of the Cottrell current.
European Patent Application 0 471 986 A2 of Tsutsumi et al. discloses a blood glucose measurement system that employs disposable sample strips. The Tsutsumi et al. system detects the presence of a blood sample by sensing a resistance across a pair of electrodes. It further employs a plurality of sensor-like strips, each having a specific resistance value which identifies it from other strips. Each of these strips has a particular application, i.e., for use during an adjustment mode of the instrument, during an error compensation mode, during a calibration mode, etc.
U.S. Pat. No. 4,999,582 to Parks et al., assigned to the same Assignee as this application, describes a biosensor electrode excitation circuit for determining if a sample strip has been properly inserted into a meter and if at least one electrode on the sample strip exhibits a proper level of contact resistance. U.S. Pat. No. 4,123,701 to Josefsen et al. also describes a dual electrode sample strip which employs a recessed well for receiving a biological sample. The instrument which receives the sample strip is provided with an opening that accommodates the sample strip and prevents its insertion in an erroneous manner. In U.S. Pat. No. 3,922,598 to Steuer et al., an electrical resistance system is described for measuring hematocrit of a blood sample. In this instance, however, an electrode probe is employed for measuring the required resistance value--rather than a disposable sample strip.
U.S. Pat. No. 4,940,945 to Littlejohn et al. describes an interface circuit for use in a biochemical sensing instrument. A disposable cartridge is employed that includes a pair of electrodes across which resistance measurements are taken. Circuitry is disclosed for sensing the presence of a fluid sample by an initial resistance measurement. In FIG. 10, Littlejohn et al. indicate that electrical contact is made to an electrode by a pair of measurement contacts so that a current flows that is sufficiently high to create a microweld--for purposes of improved electrical contact. U.S. Pat. No. 3,996,514 to Brown et al. employs plural electrodes to enable contact resistance to be measured and monitored during use of a circuit board.
Accordingly, it is an object of this invention to provide a biosensing meter with an ability to determine whether a sample strip has been properly or improperly inserted.
It is another object of this invention to provide a biosensing meter with the capability for discriminating between a sample strip and a check strip.
It is another object of this invention to provide a biosensing meter that accepts reusable sample strips and determines the quality of the sample strip upon its insertion.